1. Field of Invention
The present invention relates the field of magnetocardiogram (MCG) imaging. More specifically, it relates to reducing the noise (i.e. de-noising) of sparse measurements obtained with an electromagnetic sensor unit.
2. Description of Related Art
The field of biomagnetism generally refers to the study of magnetic fields produced by living organisms, or living tissues. For example, this field has been applied to the creation of magnetic images of the human brain and heart. Of particular interest to the present invention is magnetocardiology, the creation of magnetic images of the human heart.
Cardiac electric currents (or current impulses) are generated by electrophysiological processes in the heart. Localizing abnormal electric currents may be used in diagnosing ischemic diseases such as myocardial infarction, angina cordis, etc. It also benefits patients in the catheter lab for both treatment and follow-up, as is explained in “Forty Years of Magnetocardiology”, by F. Stroink, in Int. Conf. on Biomagnetism Advances in Biomagnetism, 28:1-8, 2010.
Traditionally, irregular cardiac electric activity, such as arrhythmia, is diagnosed by means of an electrocardiogram (ECG). However, an ECG only provides temporal information, and thus cannot localize abnormal electric impulse currents in the heart directly, even if the ischemic disease has been detected. One technique to attempt to localize electrical impulse currents is known as Body Surface Potential Mapping (BSPM), which uses a large number of electrodes (i.e., leads) to reconstruct a body surface potential map. This BSPM technique is explained in “Noninvasive volumetric imaging of cardiac electrophysiology”, by Wang et al., in CVPR, pages 2176-2183, 2009. The accuracy of BSPM electric current localization, however, is limited because the observed electrical signals can be distorted by the poor conductivity of body tissue.
The advent of the magnetocardiogram, or magnetocardiography, (MCG) made available more accurate measurements of cardiac electric impulse currents, both spatially and temporally. With reference to FIG. 1A, an MCG system consists of an MCG sensor unit 11 housing a small number of individual electromagnetic sensors 13 (typically arranged as a planar array of sixty-four or fewer sensors). Electrical impulses 17 within the body create a magnetic field 15. In the present case, the human heart 19 functions as the observed source of electrical impulses 17 (i.e. as the current source).
Each electromagnetic sensor 13 is a capture point, and hereinafter may be referenced as a capture 13. Each capture 13 measures a one-dimensional (i.e. 1D) magnetic waveform in a direction perpendicular to the sensor planar array (i.e. the z-direction) emanating from the patient's chest 21 (i.e. human torso). By aligning (or synchronizing) the depth measures (i.e. the 1D magnetic waveform) of the array of captures 13 at a given depth in the z-direction, a two-dimensional (2D) MCG map at the given depth may be constructed. The MCG sensor unit 11 is usually placed five to ten centimeters above the patient's chest 21, and measures the patient's heart magnetic field in a non-invasive manner. Thus, the array of captures 13 measure a collection of low resolution (hereinafter, low-res), two-dimensional (2D) MCG maps of electromagnetic activity.
MCG has a few advantages over ECG. First, the magnetic field generated by the heart's electrical current impulses (hereinafter, currents, electric currents or electrical currents) is not distorted in the direction perpendicular to the body surface (i.e., z direction), due to the magnetic property of body tissue. Thus MCG is more accurate and sensitive to weak electrical activity in the early stage of heart disorders. Second, the MCG sensor array can localize the position of electrical currents in the heart. Finally, MCG measurements are non-invasive. After forty years of research in MCG, cardiac electric current localization and high resolution visualization for MCG measurements are attracting more and more interest from both research and clinical areas.
However, there are a number of difficulties associated with MCG, which so far has prevented MCG from becoming a mainstream medical diagnostic tool in cardiology. A first difficulty is the great amount of electromagnetic noise that can obscure the small magnetic fields created in a human heart. This has been addressed, to some extent, by using a magnetically-shielded room to reduce background noise and by the introduction of a sensitive electromagnetic sensor 13, such as the superconducting quantum interference device (SQUID). Although these steps have helped, the raw readings nonetheless remain more noisy than desired.
Another difficulty is the limited number of electromagnetic sensors 13 that that may be housed within an MCG sensor unit 11, which limits the resolution of an MCG map. As a result, the MCG sensor unit 11 can typically produce only low resolution (low-res) 2D MCG maps. Typically, these low-res 2D MCG maps are not sufficient for localizing electric currents in the heart. For example, a 64 channel Hitachi™ MCG system with a 25 mm sensor interval (as described in “Newly Developed Magnetocardiographic System for Diagnosing Heart Disease”, by Tsukada et al., in Hitachi Review, 50(1):13-17, 2001) only measures an 8×8 MCG map (i.e. an 8×8 array of 64 measurement points).
Thus, a necessary step in MCG is generating a high resolution (hereinafter high-res) 2D MCG image, or map, from a low-res 2D MCG image, or map. Two image examples 23 and 25 of such high-res 2D MCG images are shown in FIG. 1B. Image 23 shows the tangential image of a generated high-res MCG image of a healthy heart. The maximal point (i.e. strongest point) within image 23 indicates the location (or source) of electric current in the heart. Thus, high-res MCG images permits doctors to directly “see” the electrical activity in the heart. Image 25 shows the tangential image of a generated high-res MCG image of an unhealthy heart. It differs significantly from image 23 of a healthy heart, and thus provides important cues for diagnosis. Compared to low-res MCG maps, high-res MCG images provide more diagnostic significance, and serve as the basis for an accurate electric current localization.
Most modern MCG systems use curve fitting interpolation methods between observed measurements of the electromagnetic sensors 13 to construct high-res 2D MCG images from the low-res 2D MCG maps, such as shown in “Magnetocardiographic Localization of Arrhythmia Substrates: A Methodology Study With Accessory Path-Way Ablation as Reference”, by B. A. S. et al., in Ann Noninvasive Electrocardiol, 10(2):152-160, 2005, and shown in “Evaluation of an Infarction Vector by Magnetocardiogram: Detection of Electromotive Forces that Cannot be Deduced from an Electrocardiogram”, by Nomura et al, in Int. Congress Series, 1300:512-515, 2007. Unfortunately, the accuracy of curve fitting methods is typically limited.
What is needed is an MCG system that successfully further reduces the noise in observed low-res MCG maps.
Also needed is a method of better utilizing the high-res MCG maps to improve the observed measurements of an MCG system.